During the operation of an electrochemical cell, various gases are released during the electrochemical reaction which may pressurize the case housing the cell. The pressure build-up due to the released gases can hamper cell operation thus making exhaustion of these gases important to cell operation. Relieving gas pressure is particularly important in metal-air cells. Metal-air cells include an air permeable cathode and a metallic anode separated by an aqueous electrolyte. For example, during operation of a zinc-air battery, oxygen from the ambient air is converted at the cathode to hydroxide ions, zinc is oxidized at the anode, reacts with hydroxide ions, and water and electrons are released to provide electrical energy. During this electrochemical reaction, various gases are released within the cell structure and, consequently, gas pressure increases in the structure with continued use. Because the cathode is not usually capable of supporting very high hydrostatic pressures (typically less than 2 psi), the gases generated within the cell case should be vented at low pressures to protect the cathode. While venting the gases is possible through mechanical devices, these devices must open and close and thus reseal after venting. By opening and closing a mechanical seal, the hermeticity of the battery is sacrificed which makes control of electrolyte leakage and equilibrium vapor pressure more difficult. The leakage and equilibrium vapor pressure would vary depending upon the size of the opening as well as the length of time in which the mechanical device was open. The ambient air which enters the cell through the opening may cause the metal-air cell to fail due to a condition called flooding or drying out depending upon the relative humidity of the ambient air. If the relative humidity of the ambient air is high, then the battery may fail due to flooding. However, if the relative humidity of the ambient air was low, then the battery may fail due to drying out. Also, environmental contaminants, such as carbon dioxide, may enter through an opening with a mechanical sealing mechanism.
Various structures have been implemented that vent gases generated from within a cell without using a resealing mechanical device. For example, U.S. Pat. No. 3,853,629 to Elliot, U.S. Pat. No. 3,904,441 to Badger, and U.S. Pat. No. 2,452,066 to Murphy, disclose such systems. Elliot discloses wrapper members which enclose battery cells. The wrapper members are made of an inner layer which is pervious to gases generated by the cell and an outer layer which is impervious to liquids and which is less pervious than the inner layer to gases generated by the cell. The inner and outer layers are laminated together except for a portion between the layers that serves as a passageway to vent gases generated in the interior of the battery. The inner layer serves as a mechanism to dissolve or diffuse gas in the layer. The unlaminated gas passageway is shown to open to the atmosphere at opposite exterior edges of the battery. The unlaminated gas passageway may vary in width, length and configuration depending on the application. While Elliot discloses a membrane in which gases dissolve or diffuse before exiting the cell through an unlaminated passageway, Elliot's gas exit passageway has a large surface area which exposes the inner wrapper to relatively large amounts of outside air.
Badger discloses a battery vent system particularly for use in automotive type storage batteries. Badger discloses a battery cover that has a plurality of openings which are covered by a microporous filter material. The microporous filter material is then covered by a guard member such that gas may pass laterally through the filter material to the atmosphere. In one embodiment, the gas passes up through the filter material into an elongate chamber open at both ends. Badger also exposes large areas of the filter material to the atmosphere.
Murphy discloses a gas diffusion device for storage batteries. The storage battery is made of several battery cells which each have a vent to allow the passage of gas through a porous diffusion member which may be made of sheet asbestos or sheet wool. A supplementary cover extends for some distance beyond an opening on all sides and serves to protect the diffusion member from accidental mechanical injury or deposit of dirt or other foreign matter. Murphy also exposes the diffusion member to large amounts of ambient air.
Providing a hermetic seal around leads which extend through a battery case is also important in reducing the effects that ambient air may have on a battery. In plastic cell cases, it is often a difficult manufacturing task to extend an electrode lead through an opening in the plastic case in a manner that provides a leak proof seal around the electrode. This is especially true when the case is formed in two parts joined at a seam, and one of the two electrodes of the cell is located within the battery case in a plane that is spaced from the plane in which the seam lies. In battery cells that have electrodes placed in this manner, a hole may be provided in the cell case near the electrode that is farthest from the seam of the battery case. However, it is difficult to pass the lead through the opening and provide a hermetic seal.
Various prior art structures disclose cathode leads which pass through the seam of a cell case. The cases are manufactured in a manner which provides a hermetic seal around the electrode lead. U.S. Pat. No. 3,026,365 to Hughes et al. discloses electric primary cells with cathode supports consisting of expanded or perforated nickel sheet. Each support has an outwardly directed pigtail or lead from the anode or cathode. The casing for the cell consists of pressed thermo-plastic sheets which may be polyvinyl chloride or other impermeable alkali-resistant material. The cathode is placed in a cathode casing section with its lead extending beyond the casing. Similarly, the anode with its extending lead is placed in an anode casing section. Both the cathode and anode casings have flanges around the periphery of the casing. A highly plasticized polyvinyl chloride is placed between the cathode and anode assemblies. Even pressure is imposed to the flanges of the two casing sections to form a fluid-tight and hermetically sealed assembly with the leads extended beyond the casing. Migration of the plasticizer from the membrane placed between the anode and cathode permits the flanges of the anode and cathode casing to be welded together. The flanges of the casing are then cut near the outwardly directed leads. While Hughes discloses an electrode lead extending through the seam of the cell case, Hughes does not deal with the problem of an electrode lead that is not aligned with a seam. Furthermore, Hughes requires flanges to be provided which must be cut after heating, as well as requiring a plasticized membrane between the casing sections to obtain the hermetic seal.
U.S. Pat. No. 4,664,994 to Koike et al. discloses an enclosed lead storage battery having a positive plate, negative plates and a separator and electrolyte held in position by a plate assembly. Leads for the battery assembly are coated with an epoxy resin which is then dried. A polyolefin resin having an excellent adhesiveness to epoxy resin is injection molded around the lead post so as to form a fitted doughnut shaped structure around the lead. The leads are then welded to their respective positive or negative plate. A jacket made of various kinds of synthetic resin is then heat sealed around the plate assembly and the extending leads. The jacket encloses the leads around the fitted doughnut shaped structures formed around the leads. While Koike discloses an electrode lead extending through the seam of the cell case, Koike requires that a polyolefin resin be molded around the electrode leads before sealing the leads with the case. Hughes and Koike require material additional to the casing to provide a seal around the outwardly directed leads of the cell.
Thus there is a need in the art for a vent system for an electrochemical cell which exposes the cell only to a small amount of ambient air while venting gases in a manner which maintains the hermeticity of the cell. There is also a need in the art for a cell case in which the manufacturing of a hermetically sealed electrode lead is straightforward and reliable.